By sulfur substitution of the N-5 atom in flavins and flavocoenzymes a flavin analog is obtained, Sthiaflavin, which is found to be isoelectronic and isosteric with natural flavin in the fully reduced and half-reduced states, but not in the oxidized state. Among the three 'redox shuttles' characterizing the flavin system, viz. upper le-, lower le-and 2e-shuttle, only the second one is retained in thiaflavin, which limits the redox activity of this system to le-transfer.The structure and properties of the molecular species participating in the thiaflavin redox system are discussed in comparison with the flavin system. The corresponding chemistry of a '2e-flavin', 5-deazaflavin, has been treated in the preceding paper.5-Thiaflavin is found to exhibit a stable neutral radical, which is analogous to the 'blue' flavosemiquinone. Unlike normal flavin, where the radical is in a dismutation equilibrium, thiaflavin radical shows reversible formation of a covalent dimer, which is stable in aprotic solution and disproportionates only in water, with irreversible formation of a sulfoxide. The ultraviolet and infrared spectra of the dimer are in agreement with the structure of two 5-thiaflavin molecules linked covalently at their 4a carbons. This corroborates the earlier hypothesis that the essential intermediate in the dismutation of normal flavin is likewise a covalent dimer.Thiaflavin is tightly bound by apoflavodoxin. The protein catalyses the autoxidation to the radical state. Thiaflavodoxin radical is even more stable (towards further oxidation) than is the free thiaflavin radical.The redox potential of the couple reduced thiaflavin/thiaflavin radical (sFlred/sF1 3 is surprisingly high. From the reversible equilibrium established with ferricyanide, sFlred + Fe(CN)g-+ sF1' + Fe(CN)%-, the standard potential of the sFlre&F1' couple, Em at pH 7, has been estimated as + 0.38 V.Recently, a modified type of flavocoenzyme, i.e. 5-deazaflavin [l-61, has been the subject of a steadily growing interest. In reviewing the available data on 5-deazaflavoproteins [7], we have reiterated the principles characterizing flavin-dependent redox catalysis, which were established earlier [8].In contrast to the mandatory 2e-transfer agent nicotinamide, natural flavin appears to be ambiguous by its capability to catalyze le-as well as 2e-transfer.In addition to 5-deazaflavin, which is a flavin analog lacking le--transfer activity [l], we have searched for a modification of the flavin nucleus which would remove the 2e--transfer activity in' favor of Abbreviations. sFl,,dH, thiaflavin (reduced state); sF1, thiaflavin radical; sFI& thiaflavin sulfonium cation; HsFl,,-tO, thiaflavin sulfoxide; Fl,,, oxidized flavin; HFl, flavin radical; H2Flred, reduced flavin; (sFl)z, thiaflavin radical dimer ; (HF1)2, flavin radical dimer ; EtrC2H5-;. flavin = l0-alkylated-7,8-dimethylisoalloxazine; s5FMN, ribo-5-thiaflavin 5'-monophosphate; NMR, nuclear magnetic resonance; EPR, electron paramagnetic resonance. le-transfer. Such a derivative has been found by rep...

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1 0-w-Dime thylaminopropyl-dioxopyrimido[ 5,4-b] [ 1,4] benzo thiazin ein Promazin-analoges Thiaflavin') -Die Synthese eines N (10)-Dimethylaminopropyl-substituierten 5-Thiaflavins 2c erfolgt durch Kondensation von 5-Chlorbarbitursaure mit dem ent sprechend substituierten 2-Aminothiophenol, welches durch Alkylierung und Hydrolyse von Benzothiazolin-2-on erhalten wird. Spektrale Eigenschaften, Azidobasizitat und Charakteristika des Thiaflavin Redoxsystems im Vergleich zu Phenothiazinen werden diskutiert. Aufgrund der Struktur-Analogie zu Flavinen und pharmakologisch interessanten Phenothiazin-Derivaten wird 2c als Model1 fur die Untersuchung der Interaktion von Flavocoenzymen mit Neuroleptica vom Chlorpromazin-Typ vorgeschlagen. l0-w-Dimethylaminoprpylpyrimido[S,4-b] [ 1,4]benzothiazinedione, a Romazin Analogue of Thiaflavin N( l0)-Dimethylaminopropyl-5-thiaflavin is prepared by condensation of the corresponding 2-aminothiophenol, formed from benzothiazolin-2-one by allcylation and hydrolysis. The spectral behavior, the acidbase properties and the phenothiazine-lie thiaflavin redox system are discussed. Because of its structural similarity with flavins and phenothiazine derivatives of pharmacological interest, 2c is regarded as a model compound for studies about the mechanism of action of neuroleptic drugs related to chlorpromazine. Substitution des N(5) im Flavin 1 durch Schwefel fuhrt zum 5-Thiaflavin 2. Struktur und Eigenschaften dieses Redox-Systems belegen dessen Verwandtschaft zu Flavinen und Phenothiazinen 3. Wghrend die reduzierte Stufe dem Dihydroflavin isoelektronisch ist, verhalt sich das Pyrimidobenzothiazoniumkation phenothiazinanalog, indem es sich in wassrigem Milieu zum Sulfoxid 10 stabilisiert2-6). Die semi-Die Charakterisierung der semichinoiden Radikale hat auch einen Bezug zu der Reaktivitat der Thiaflavine im biologischen System. So wird fur Flavoprotein-Interaktionen der Phenothiazin-Radikalkationen eine hohere Aktivitat na~hgewiesen'~). 3 Formelschema 4 14 L. Levy und T.N. Burbridge, Biochem. Pharmacol. 16, 1249 (1967).

lO-Alkylpyrimido[5.4-b][ 1.4]benzothiazine 10 (5-Thiaflavine) entstehen aus den durch Umsetzung von Anilinouracil-Derivaten 3 mit SOCI, erhaltenen 4a-Chlor-pyrimidobenzothiazinen 8 durch Reduktion mit Hydrazin. Die als Intermediarprodukte bei der Bildung von 8 postulierten Sulfoxide 6 lassen sich auch aus 8 durch Hydrolyse erhalten. Erneutes Einwirken von SOCI, auf 6 fuhrt wieder zu 8. Als Nebenprodukt dieser zu einheitlichen Isomeren fuhrenden Thiaflavin-Synthese entsteht das Pyrimido[4.5-b]indol 9, fur dessen Entstehen die Zwischenstufe des 5-Chloruracils 5 plausibel ist. 1,4]benzothiazines 10 (5-thiaflavines) were prepared by hydrazine reduction of 4a-chloropyrimidobenzothiazines 8 formed from the anilinouracils 3 with SOCI,. The postulated intermediates of this reaction, the sulfoxides 6, were prepared by hydrolysis of 8. Reaction of 6 with SOCI, yields 8. In addition to 8, the pyrimidoindoles 9 are formed by cyclization of the 5-chlorouracils which arise in the chlorination of 3. A New Synthesis of 5-Thiaflavine Derivatives lO-Alkylpyrimido[5,4-b][

Donator-substituierte 5-Desazaflavine 7 werden mit Hilfe einer Vilsmeier-Reaktion aus Anilinouracil-Derivaten 5 iiber die isolierbare Zwischenstufe 6 dargestellt. Der spektroskopische Vergleich der 5-Desazaflavine 7 zeigt, daR die Konjugationsfahigkeit der Substituenten in der Reihenfolge: 8-CH,, 7-OH, %OH,,, und 8-0-zunirnmt. Die 8-Hydroxy-Verbindung 7b ist Modellsubstanz fur ein natiirliches 8-Hydroxy-5-desaza-FMN, welches in methanogenen Bakterien aufgefunden wurde. 7b und das 8-Dimethylamino-Derivat 7k sind isoelektronisch zu entsprechenden Flavocoenzymen mit 8-Hydroxy-und 8-Dimethylamino-Funktion. Donator-substituted 5-Deazaflavins, 119 Hydroxy-and 8-Dimethylamino-5-deazaflavins -Model Compounds of Naturally Occuring (Deaza-)FlavocoenzymsDonator-substituted 5-deazaflavins 7 were prepared by a Vilsmeier procedure from the anilinouracil derivatives 5 via 6 as stable intermediates. Spectroscopic comparison of the 5-deazaflavins 7 shows, that resonance of the substituents increases in the sequence: , &OH,,, and 8-0-. The 8-hydroxy derivative 7b is a model compound of a natural 8-hydroxy-5-deaza-FMN, which was found in methanogenic bacteria. Compound 7b as well as the 8-(dimethylamino)deazaflavin 7k are isoelectronic with the flavocoenzymes with 8-hydroxy and 8-dimethylamino functions.